Technology
Retinal Analysis Using the Confocal Scanning Laser Ophthalmoscope (cSLO)
The Heidelberg Retina Tomograph (HRT) is a confocal scanning laser. A laser light scans the retina in 24 milliseconds, starting above the retinal surface, capturing parallel image sections at increasing depths, which can be combined to create three-dimensional images of the retina. Two types of analyses are available: the Edema Index and Retinal Thickness (including retinal volume). Images are aligned and compared using TruTrackTM technology for both individual examinations and for detecting progression between examinations. The same technology is used in the HRT Glaucoma Module.
Edema Index
The Edema Index is a black and white map of the retina which can reveal areas of increased fluid accumulation even prior to observing clinically significant macular edema. The HRT uses a beam of light to scan a 15-degree region of the retina. Reflected light from the retina returns a characteristic signal. If the signal is disrupted, then the area is highlighted. Disruption of the reflected light signal is often associated with fluid accumulation. Characteristic patterns of pathology can be seen in the images, similar to the patterns seen in fluorescein angiography.
However, the information captured by the Edema Index is very different than fluorescein angiography. Instead of tracking dye in the blood vessels, the software detects the vitreo-retinal interface and then looks for an “expected” penetration and reflection of light from tissue. When increased interstitial fluid is present, it “stretches” the light signal. The more the signal is stretched, the higher the Edema Index value. Higher values are reflected back bright and white, and lower values are reflected back dark or gray. White areas are indicative of fluid accumulation.
The Edema Index number is a relative value. An expected value based on an expected signal return is 1.0. Values higher than 1.0 indicate disruption of the signal and as the value approaches 1.8 or more, it typically correlates with fluid accumulation.
Retinal Thickness
The HRT provides retinal thickness measurements with discrete millimeter measurement. Unlike the Edema Index, the Retinal Thickness function locates both the vitreo-retinal interface and the retinal pigmented epithelium (RPE).
A beam of light scans the retina and two major signal peaks are detected. The largest signal peak corresponds to the internal limiting membrane (ILM) and the second largest signal peak corresponds to the RPE. The HRT then calculates the difference between the two peaks to arrive at the millimeter difference.
The HRT scans a 15-degree area using 384 x 384 pixels or nearly 150,000 measured points across the measurement area. Other technologies use cross-sectional imaging which only “samples” the area and interpolates the values between scan lines. HRT data is not interpolated.
24 Millisecond Scan Helps Avoid Eye Movement Artifact
The HRT scans the retina in 24 milliseconds, faster than most involuntary eye movements (micro saccades occur in 30-50 milliseconds) and faster than voluntary eye movements (macro saccades occur in 150 or more milliseconds). Each scan is composed of 384 x 384 pixels for a total of 147,456 data points, covering a 15-degree area of the retina. Other technologies, such as time-domain OCT, scan slower than 24 milliseconds and interpolate data from only 768 data points. Eye movement can cause motion artifact in slow scanning systems (blurring of the data).
TruTrack™ Image Alignment Provides Accuracy in Single Exams
TruTrack™ is our proprietary software which aligns images within examinations and between examinations. The software uses anatomical features (such as blood vessel patterns) and other image characteristics to align the images. Good image analysis requires both high quality images (fast scanning) and image alignment. To construct a single examination, the software aligns a stack of individual scans. Each scan is examined for image quality and if any are found to be outside of acceptable quality indices, the scan is discarded from the image set. Three sets of scans are automatically captured at each acquisition, providing data redundancy. The three sets are used to create a final composite image for analysis. Only Heidelberg Engineering offers retinal imaging technology which automatically discards low quality scans.
TruTrack™ Image Alignment Detects Patient Variability and Progression
TruTrack™ software helps align images both during an examination and between examinations. When comparing examinations, the software looks at over 500 image points, using blood vessel patterns and image characteristics to match, compare and align images. The 10 most prominent points are highlighted onscreen and the images toggle back and forth, allowing the operator to assess the quality of the alignment. If desired, the user can adjust the 10 marked points and override the automatic image alignment.
Images can only be aligned if they overlap the same area of the eye by at least 25%. In the event the software cannot align the images (due to insufficient overlap, poor quality, etc.), an error message will be displayed. In the majority of cases, the software is able to align the images.
Once the images are aligned, the software can measure differences between the images. For a single examination, the software can determine patient-based variability based on a set of scans within the single examination. Knowing within-examination variability helps rule out measurement differences due to patient variability. If an area of change exceeds the within-examination variability from both the baseline and the follow-up examinations, then the area is flagged. It is both precise alignment and careful elimination of patient variability that enables the HRT to provide a strong indicator of change.